Chlorine dioxide is utilized in a variety of bleaching operations, particularly in the bleaching of cellulosic fibrous material, such as, wood pulp. In our prior U.S. Pat. Nos. 3,895,100 and 3,975,506 (E107) the disclosures of which are incorporated herein by reference, there is disclosed a chlorine dioxide generating process and equipment therefor wherein chlorine dioxide is formed by reduction of an alkali metal chlorate, usually sodium chlorate, with chloride ions in an aqueous acid reaction medium, in accordance with the equation: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O
The reaction medium in the generator is maintained at its boiling point, generally about 25.degree. to about 90.degree. C., under a subatmospheric pressure, generally about 20 to about 400 mm Hg, to cause the deposition of a by-product salt from the reaction medium in the generation vessel once saturation has been achieved after start-up and to remove the chlorine dioxide and chlorine in gaseous admixture with steam.
The by-product salt which is precipitated depends on the acid used in the reaction medium, the by-product being a sodium sulphate when sulphuric acid is used and sodium chloride and/or hydrogen chloride provide the chloride ion reducing agent, and the by-product being sodium chloride when hydrochloric acid is used and also provides the reducing agent.
When sulphuric acid is used, the sodium sulphate may be in an acidic or neutral form depending on the total acid normality of the generator, with a neutral salt generally being obtained at total acid normalities of about 2 to about 5 normal and acid salts being obtained at higher values usually up to about 12 normal.
When the total acid normality of a sulphuric acid based system is such as to produce neutral sodium sulphate, it is preferred to use a reaction temperature above about 30.degree. C. in order to obtain the anhydrous salt.
When hydrochloric acid is used, the actual hydrogen ion concentration in the reaction medium is maintained in the range of about 0.05 to about 0.3 normal. The term "actual hydrogen ion concentration", as used herein, is the value determined by a pH meter calibrated on an 0.1 normal solution of hydrochloric acid on the assumption that such a solution is 100% dissociated at this concentration.
As described in our earlier patents, the solid by-product is removed as a slurry in reaction medium from the generator vessel, sodium chlorate solution is mixed therewith, the mixture is recycled through a reboiler wherein the mixture is heated to the reaction temperature and forwarded to the reaction vessel after the addition of acid to the heated mixture. A portion of the slurry is withdrawn prior to the reboiler for crystal separation.
The heated mixture from the reboiler is passed through a venturi-like pipe which exerts sufficient back pressure on the mixture in the reboiler to prevent boiling therein, acid in concentrated form is introduced to the collar of the venturi and the resultant reaction mixture is allowed to expand at low acceleration to permit boiling and gaseous product formation to occur evenly and gradually and thereby avoid bumping and vibration, so that a fluid mixture of soild phase, liquid phase and gaseous phase enters the generator above the liquid level therein through a curved pipe joining the downstream end of the venturi-like pipe and the generation vessel inlet.
While this prior art procedure represents an excellent commercially-viable chlorine dioxide generating system having many advantages over prior art arrangements, as described in our earlier patents, it does suffer from the drawback that it is not capable of modular manufacture, transportation and installation but rather requires considerable on-site assembly of the reboiler, or other suitable heat exchanger, and recycle pipes. In addition, considerable lateral space beyond the dimensions of the generator vessel is required to accommodate the reboiler and recycle tubes.